Known rubber injection molding machines commonly include a press or clamping unit for a mold and an injection unit for injecting uncured rubber into the mold. The mold typically includes upper and lower mold halves or parts which are heated and clamped together under high pressure by the clamping unit. The injection unit has an injection nozzle through which the uncured rubber is injected into the mold for cure under pressure and high temperature conditions. Rubber injection molding machines of various press clamping forces and injector shot capacity are manufactured by U.S. Molding Machinery Company of Mentor, Ohio.
In a typical machine cycle, the mold is clamped in the clamping unit and then uncured rubber is injected into the mold by the injection unit. The rubber is then cured (vulcanized) in the mold for a prescribed time after which the press is opened for removal of the molded rubber product from the mold. After this the press is again closed to commence another machine cycle and production of another molded rubber product.
During the curing and/or product removal phases of the machine cycle, the injection unit may be recharged for the next machine cycle. The recharging time often is considerably shorter than the time needed to cure the rubber and remove the cured rubber product. Consequently, the injection unit may be idle for a substantial portion of the total cycle time. As has been recognized in the art of rubber injection molding for some time, advantage can be taken of this idle time in relation to one mold by using the injection unit to inject a charge into another mold or molds.
One known rubber injection molding machine includes a single injection unit which may be shared by up to six press units in respective stations of the machine. The press units are arranged in a row and the injection unit is shifted laterally from station to station thereby sequentially to inject a charge into the mold clamped at each station. This technique of moving the injection unit from one station to another is subject to several drawbacks. One drawback is the relatively high cost and complexity of the mechanism and controls needed to shuttle the injector unit between stations. The moving injector unit also creates concerns about misalignment between the injection unit and the molds which may cause such problems as leakage and excessive wear.
Another known rubber injection molding machine includes a single station press equipped with traveling carriages for two molds and associated hardware. When the press is opened, one mold is shifted out of the press to an ejector station at one side of the press while the other mold is shifted into the press from an ejector station at the opposite side of the press. The press is then closed and a charge is injected into the mold by an injection unit mounted atop the press. This press has advantage in those applications where long open times are needed or considerable clearance is required for stripping.